The mechanism of action of epidermal growth factor (EGF) on liver will be investigated in conjunction with experiments to elucidate the function of this polypeptide hormone in the overall growth and development of the organism. Since EGF has been shown to act in combination with insulin and glucagon to stimulate hepatic proliferation in intact adult animals, it is planned to investigate the nature of this synergism and its role in the regulation of cellular growth and differentiation. A homogeneous population of adult rat liver parenchymal cells maintained as nondividing primary cultures will be used as a model system to investigate the mechanism of action and biological effects of EGF on the liver. We have determined that 125I-labeled EGF is capable of binding to these cultured hepatocytes in a specific and saturable manner. In order to further elucidate the mechanism of action of EGF, studies will be conducted to determine if the membrane-bound 125I-EGF acts only on the external surface of the membrane, or whether it is capable of entering the hepatocyte and being metabolized. Several parameters will be evaluated to assess the biological responses which occur following binding of EGF to the cell. The rates of synthesis of RNA, DNA, and protein will be measured as well as the activities of certain key enzymes intimately associated with DNA synthesis. The effects of EGF on membrane transport of amino acids and uridine also will be determined. These studies are designed to provide deeper insight into the mechanism of action and biological effects of epidermal growth factor on liver. In addition, the proposed investigation of the synergistic actions of EGF with pancreatic hormones will provide extensive information regarding the complicated series of biochemical events which regulate processes such as liver regeneration.